The operation of cupolas with basic slags has, as its principal objective, the production of low sulfur and/or high carbon irons. The basic cupola is used extensively for making base iron for nodular iron because low sulfurs are easily obtainable along with the higher carbon levels that are typically desired. One of the disadvantages of using such a basic slag operated cupola is the cost of fluxing material which typically exceeds that of an acid operating cupola. Fluxes should lower the fusion point and improve the fluidity of the slag naturally produced in the melting operation; the fluid condition of the slag will influence physical cleanliness, the various reactions, and the combustion efficiency of the cupola. Although a natural slag is formed by non-metallic products such as coke ash, dirt or entrapped sand obtained in the metal charge, and oxidized metallics from melting operations, the properties of the natural slag will be altered by the addition of fluxing agents, such as limestone, which ultimately becomes part of the slag.
A flux is here used to mean a material that reacts with the natural slag to increase the fluidity and refining value thereof. Limestone is vital in controlling the desulfurization reaction in the basic operated cupola. Within certain limits, limestone additions depress the slag fusion point, but excess limestone will increase the slag fusion point. Furthermore, the slag fusion point will increase in operations that strive for strict control of low sulfur levels in the base iron composition. Therefore, special or secondary fluxing agents (referred to as fluidizers) have been resorted to in an effort to reduce the slag fusion point and accelerate the solution of lime thereby insuring the occurrence of required basic slag refining reactions.
To do this, the prior art has turned principally to the use of fluorspar usually added in the form of calcium fluoride. This material has proved capable of providing a highly fluid slag. However, with the advent of stricter environmental restrictions on emissions from a cupola, it has been found that hydrofluoric acid, formed as the reaction gas from the use of calcium fluoride, deteriorates fiberglas-type bags utilized to collect the residue and particles in the effluent. Hydrofluoric acid gas will fog and deteriorate the effectiveness of the collection elements much earlier than their normal expectancy. Additionally, the cost of calcium fluoride has risen to unprecedented heights, causing cupola operators to turn to more economical substitutes that will not only perform well as the secondary fluidizer but eliminate the problem relating to baghouse collection.
Unfortunately, there has been no available alternative fluidizers that would meet the triple goals of (a) achieving greater economy compared to fluorspar, (b) improving fluidity by decreasing the fusion point of the slag and thereby be equivalent to the effectiveness of fluorspar, and (c) the elimination of the bag-house problem. Since these triple goals cannot be solved simultaneously by the knowledge of the prior art to date, the present invention has undertaken to re-analyze the function and capabilities of traditional materials in proportions heretofore not used.